Single carrier Frequency Division Multiple Access (SC-FDMA) is currently the standard uplink transmission scheme in 3GPP Long Term Evolution (LTE) mobile telecommunication systems. SC-FDMA can be viewed as a Discrete Fourier Transform (DFT) pre-coded Orthogonal Frequency Division Multiple Access (OFDMA) wherein a signal being transmitted in the uplink (UL) direction from a User Equipment (UE) to a Base Station (BS) will be first transformed into the frequency domain via an N point DFT and the DFT output will be assigned to a number of selected subcarriers within one single carrier for transmission. The main advantage of SC-FDMA is that it allows for a lower Peak-to-Average power ratio (PAPR) compared to that of OFDMA for low order modulation schemes like Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), which benefits the mobile UE in terms of power efficiency [1]. However, this requires the selected sub-carriers to be consecutive or equidistantly distributed in the entire bandwidth of the single carrier [2]. These sub-carrier selection methods are usually referred to as localized FDMA (LFDMA) [3] and interleaved FDMA (IFDMA) [4], respectively. It has been shown that IFDMA has the lowest PAPR [3] which also provides large frequency diversity, while LFDMA can benefit more from frequency dependent scheduling to improve the overall data throughput within a SC-FDMA mobile telecommunication system [5]. Another variant of DFT pre-coded OFDMA using regularly interleaved blocks of sub-carriers is denoted Block-IFDMA (B-IFDMA), which provides robustness to frequency offsets at the expense of increased PAPR compared to IFDMA [6].
In a SC-FDMA telecommunication system, wherein both UE and BS have at least two antennas, there are two uplink radio transmission scenarios, which are briefly presented below under item A) and B).
(A)
Single-User MIMO
One possibility of the uplink transmission strategy within a SC-FDMA telecommunication system is to use the so called single-user MIMO technique. In this configuration, each UE accesses the radio channel in a (Time Division Multiple Access (TDMA) mode. Within the assigned Physical Resource Blocks (PRBs) for radio transmission of one time slot, only one UE is active and it can use spatial multiplexing technique to transmit up to nt independent data streams from different antennas. Thereby, nt, which is the number of transmit antennas of the UE, is smaller than the number nr, which is the number of receive antennas of the BS (nr>nt).
This Single-user MIMO scheme has several disadvantages:
(1) Due to the typically limited space of the UE, the transmit antennas mounted in the UE are likely to be highly correlated. As a consequence, data streams from different antennas may be harder to separate by the BS.
(2) A small singular value of the channel matrix H in one PRB, which is equal to a rank deficiency of H in that PRB, will degrade the rate of the whole data throughput because the transmit power will have to be drained in that PRB. This can be understood because DFT pre-coded OFDM is equivalent to an equal gain power allocation. Thus the use of “bad” PRBs is reducing the spectral efficiency dramatically compared to OFDM without a DFT pre-coding. Consequently, only less data streams can be supported.
(B)
Multi-User SDMA with Using Only One Antenna Per UE
The second possibility of the uplink transmission strategy is that each UE uses only one antenna to transmit a single data stream and up to nr data streams from different UEs can be multiplexed in space which can be separated at the BS. The technique is also called multi-user SDMA or virtual MIMO technique. Thereby, similar to OFDMA systems, SC-FDMA is combined with Spatial Division Multiple Access (SDMA) by applying spatial processing on a sub-carrier basis, in the frequency domain after DFT pre-coding to improve the data throughput within the telecommunication system. This technique enables independent data streams from different user equipments (UEs) to use the same sub-carriers to communicate with the BS simultaneously. At the BS receiver, simple MIMO equalization such as for instance zero-forcing (ZF) equalization can be carried out in the frequency domain to separate the data streams from different UEs, which are then transformed back to the time domain via Inverse Discrete Fourier Transform (IDFT) for data decoding and detection. However, in a telecommunication system with UEs each employing a single antenna only (further antennas are switched off), the compound channel between the transmit antennas of the UEs and the receive antennas at the BS will be close to rank deficiency at some sub-carriers if the UEs have similar spatial signatures at those sub-carriers. Moreover, even the UEs can be well separated in spatial domain but they may experience deep fades at some sub-carrier due to the frequency selectivity of the mobile channel. Unlike OFDMA systems, those sub-carriers cannot be excluded from transmission due to sub-carrier mapping constraints. Consequently, the compulsory usage of those sub-carriers will lead to significant loss of the overall data throughput within the telecommunication system [7]. This holds in particular if the UEs occupy large bandwidth, which is a typical case in future broadband wireless telecommunication systems.
Specifically, the virtual MIMO technique described under item (B) can mitigate the first disadvantage (1) of the single-user MIMO UL radio transmission technique by exploiting the degrees of freedom in user grouping also known as multi-user diversity, but it cannot overcome the second disadvantage (2) of a rank deficiency of the channel matrix H on some of the PRBs.
There may be a need for providing an uplink radio transmission strategy which allows for a large overall data throughput within a multi-user MIMO SC-FDMA telecommunication system.